1. Field of the Invention
The present invention relates generally to a video disc player, and more particularly, to a high-definition video disk player containing a clamping circuit for accurately clamping a clamp level signal multiplexed on a MUSE signal reproduced from a video disc at predetermined timing.
2. Description of the Background Art
In recent years, a high-definition video disc player for optically reproducing high-definition video information recorded on a video disc has been developed. In general, a high-definition video signal is subjected to bandwidth compression by a TCI (Time Compressed Integration) multiple sub-nyquist sampling encoding system, i.e., a MUSE converting system, to be recorded on such a video disc. Such a MUSE converting system is described in an article by Y. Ninomiya et al., entitled "BROADCASTING AND TRANSMISSION SYSTEM-MUSE", Technical Papers of Third International Colloquium on Advanced Television systems; HDTV 1987, Ottawa, Canada, Oct. 4-8, 1987. A so-called MUSE signal obtained by such a converting system is suitably used in a transmission system and a recording/reproducing system of a high-definition video signal.
The MUSE signal thus recorded on the video disc is optically reproduced by the high-definition video disc player and further applied to a MUSE decoder. The MUSE decoder converts the reproduced MUSE signal into a high-definition video signal, to supply the same to a display device for high-definition video, or the like.
According to the standard of the MUSE signal, a clamp level signal is multiplexed on the 563rd horizontal scanning period (line) and the 1125th line in one frame of the MUSE signal so as to define a DC intermediate level of the reproduced MUSE signal. Such a clamp level signal is digitally processed in the MUSE decoder, to be accurately clamped. FIG. 2 is a diagram showing the arrangement corresponding to one frame of the MUSE signal according to the above described standard. The MUSE decoder analog-to-digital converts the reproduced MUSE signal and further separates horizontal synchronizing signals. When the MUSE decoder detects first and second frame synchronizing pulses at the beginning of each frame, the MUSE decoder starts counting of the above described separated horizontal synchronizing signals, generates clamp pulses at the timing of the 563rd line and the 1125th line and accurately clamps the clamp level signal on the above described particular lines, to define the DC intermediate level (128/256) of the reproduced MUSE signal to a predetermined level.
In general, the above described clamping operation is digitally performed in the MUSE decoder. Thus, the video disc player itself does not generally contain such a clamping circuit.
However, when a digital time base corrector (compensation) is provided in the video disc player to attempt to make jitter correction of the reproduced MUSE signal, the following problem occurs because no clamping circuit is provided in the video disc player itself as described above.
More specifically, if the above described clamping operation is performed only in the MUSE decoder in the succeeding stage of the video disc player, a DC level of a MUSE signal entering the video disc player remains unstable. Consequently, a conversion range of an A/D converter in the first stage of the time base corrector must be wider than a range in which the entering signal itself fluctuates because vertical fluctuations in the DC level added to fluctuations of the entering signal itself must be considered, resulting in the decrease in resolution of analog-to-digital conversion.
Accordingly, if a clamping circuit is provided in the video disc player to accurately clamp the entering MUSE signal, fluctuations in the DC level of the entering MUSE signal can be prevented. Thus, the conversion level of the A/D converter in the first stage of the time base corrector need not be increased to a level more than necessary, so that converting efficiency can be increased.
Meanwhile, separation and counting of horizontal synchronizing signals to be a clue to the clamping operation as described above are digitally accomplished in the MUSE decoder. However, such digital processing means is not provided in the video disc player. Thus, there is a problem of the necessity of adding a very complicated circuit in the video disc player so as to perform the clamping operation of the MUSE signal.